A 1,000 microwave will draw between 1,300 and 1,500 watts, possibly more than the air conditioner. If you have a solid 15 amp supply, you should be able to run the microwave, air conditioner, or water heater one at a time, possibly needing to switch the fridge off or to gas while running any of these 3 items. Use a good "dogbone" style 15-to-30 amp adapter, not a "hockey puck" type. Monitor the AC voltage inside the RV. If it goes below about 105, your AC supply isn't up to supporting the load. If it stays above 110 IMO you're ok. In-between is the "gray area".